ACFM to SCFM Calculations

[UtilityLouie]

Hi,

I need help understanding the ACFM to SCFM relationship. It seems basic to me, but I think I am getting turned around.

I am doing work for a paper company and I am doing a building air balance in a paper machine room. There are a large amount of vacuum pumps that draw from this room. Let's say that I know the vacuum pumps are sized for 160,000 ACFM at -21" Hg of vacuum at the pumps. How many SCFM is this?

I have seen this calculation related to density and also temperature and pressure (without considering water vapor). The air at the pump will be moist. What the moisture is, I don't know. I am sure the air at the suction point on the machine is saturated, but at the pump the water vapor would have some super heat due to the drop in pressure. The density of dry air to that of saturated air or very wet air is very different.

Help is appreciated.

 

[prodeng]

I have a little booklet from Roots division of Dresser ((317) 827-9200), dated 1988 that addresses your question. A simple formula is listed with examples. I will try to abbreviate it, but clearly state it as well.

ACFM=SCFM * [Ps-(RHs x Pvs)/(Pb-(RHa x Pva)] * Ta/Ts * Pb/Pa

where Ps is standard pressure (psia)
Pb is atmospheric pressure (psia) - barometric reading
Pa is actual inlet pressure (after any pressure drops through filters, etc.)
RHs is standard relative humidity
RHa is actual relative humidity
PVs is saturated vapor pressure of water at std temp.
PVa is saturated vapor pressure of water at actual temp.
Ts is standard temperature in deg. Rankine (=F+460)
Ta is actual temperature (inlet as well)

It has a chart giving saturated vapor pressure at various temperatures as well, but I can't reproduce it all here. The chart ranges from 32 F to 110 F, and is not linear (no surprise there).

In case you don't have STP handy, the standard conditions they use in their examples are Ps=14.7, RHs=.36, PVs=.3391, and Ts=528.

I know this booklet helped me numerous times in the past. If you need a saturated vapor pressure for a certain temperature, post a note here and I will look it up in the booklet for you.

 

[FSarbu]

Hello everyone,
I need help in dimensioning a minimum size opening so that a certain cfm of air will pass through when subject to a given pressure.
Thank you.